Aging effect on the structure formation of active sites in single-atomic catalysts and their electrochemical properties for oxygen reduction reaction

Abstract

FeNC catalysts have attracted attention because of their superior oxygen reduction reaction (ORR) performance in alkaline media. However, there is a conflict between the active sites of FeNC catalysts, namely single-atomic Fe–Nx sites and dual active sites consisting of atomic sites/iron carbide nanoparticles. Herein, we synthesized catalysts with different types of active sites by controlling the facile aging process. The main active sites varied from single active sites to dual active sites when the aging time and Fe content was decreased and increased. A catalyst with 0.6 wt% Fe that is aged for 24 h (FeNC-24–0.6) has dual active sites, whereas a catalyst with 0.9 wt% Fe that is aged for 48 h (FeNC-48–0.9) consists mainly of single active sites. The catalysts showed outstanding ORR activity, exceeding the half-wave potential of the commercial 20 wt% Pt/C catalysts by 11 mV. Interestingly, the FeNC-48–0.9 catalyst exhibited a rare negative shift compared to the FeNC-24–0.6 catalyst in the durability tests. Therefore, it is believed that increasing the number of single-atomic Fe–Nx sites is an effective approach to enhance the ORR performance of FeNC catalyst in alkaline media.